The present invention relates to superconducting generators, and more particularly to a rotor structure for a generator having a superconducting field winding.
Large alternating current generators with superconducting field windings offer great potential benefits. Such machines operate with no losses in the field winding itself and thus can provide higher magnetic fields than conventional generators and with greater efficiency. This allows the generation of more power per unit volume of the generator and thus makes it possible to achieve greatly increased ratings or to reduce substantially the size and cost of a machine of given rating. The reduction in electrical losses during operation of the machine is also an important advantage and represents a very substantial saving in operating cost.
Such machines have rotating field members consisting of a cylindrical rotor carrying the superconducting field winding. The winding conductors are made of a superconducting material and means are provided for circulating a cryogenic coolant fluid during operation of the machine to maintain the windings in the superconducting state, that is, at a temperature below the critical temperature of the material. An external refrigeration system is provided for this purpose to supply liquid helium to the winding during operation, the helium exhausting in the gaseous state for return to the refrigerator.
Such a rotor preferably has a relatively massive cylindrical outer rotor member which operates essentially at ambient temperature, and a separate inner rotor member carrying the field winding itself and operating at cryogenic temperatures. The rotor is preferably evacuated to minimize heat flow to the superconducting winding and may be provided with radiation shields to further reduce heat transfer. The inner rotor member is supported on the outer rotor member and the inner and outer members must be connected together in a manner to enable the transmission of torque during operation of the machine. The inner rotor member carrying the field winding operates at superconducting temperature, however, while the outer member is essentially at ambient temperature, so that there is an extremely large temperature difference between the two members during operation. A large differential thermal contraction of the inner member relative to the outer member, therefore, occurs during the initial cool-down of the rotor at the beginning of operation, and a corresponding expansion occurs after shutdown when the rotor is allowed to return to room temperature. It is necessary, therefore, not only to provide for the transmission of torque but to allow this large differential thermal contraction to take place without introducing serious stresses or distortions in the structure.
It has been proposed heretofore, as disclosed in a copending application of G. D. Cooper et al, Ser. No. 734,943, filed Oct. 22, 1976, now U.S. Pat. No. 4,123,676 and assigned to the Assignee of this invention, to provide for relative sliding movement between inner and outer rotor members to allow for thermal contraction. For this purpose, cylindrical engaging surfaces are provided on the inner and outer members at one end of the rotor and a driving connection is made only at the other end. The inner and outer members are thus free to slide on each other to permit the differential contraction and expansion. Such a support, however, requires very accurate control of the friction between the engaging surfaces to permit sliding when necessary while providing proper support during normal operation. This requires careful design and manufacture, and extensive testing. Furthermore, such a support is preferably located outside the vacuum enclosure of the inner rotor member so that a vacuum seal of bellows type is required to accommodate the large differential expansion, which is undesirable.